Structural variation of the anterior and posterior anulus fibrosus in the development of human lumbar intervertebral disc. A risk factor for intervertebral disc rupture.

A morphologic comparison of the laminated structure of the anterior and posterior anulus fibrosus of lumbar intervertebral disc was made on 24 L3-4 and L4-5 discs obtained from 10 fetus, one child, and one young adult cadavers. Low power microscopic and polarized microscopic observations and measurements on the transverse sections were performed to assess 1) lamellar bundle arrangement, 2) number of distinct lamellar bundles, 3) percentage of incomplete lamellar bundles, and 4) fiber-interlacing angles. The number of lamellar bundles was greater in the anterior anulus than in the posterior anulus. A very complex structure was observed in the posterior middle anulus in all the specimens, with a high percentage of incomplete/discontinuous lamellar bundles and greater fiber-interlacing angles. Loose connection of the lamellar bundles of posterior outer anulus was observed in fetal discs. The annular strength was not measured in the present study, however, the risk of posterior disc rupture in the young may be influenced by an inherent structural variation with a weak posterior anulus fibrosus.